AAPG ASUSC - LET’S LOG IN 1

SPECTRAL GAMMA LOG Spectral gamma logs record individual responses for K-, Th– and U– bearing minerals. The detectors record ra-diation in several energy windows as GR-K, GR-U and GR-Th. Applications: 1. Clay content evaluation (spectral logs will distinguish between clays and other radioactive minerals like phos-phate. 2. Clay type identification (ratios such as Th:K are used to identify particular clay minerals). 3. Source rock potential (there is an impirical relation-ship between U:K ratios and organic carbon in shales). *spectral gamma sondes also provide a total GR count that is equivalent to a conventional gamma log. DIFFUSED GAMMA-RAY LOGS A gamma source is used to bombard the formation and the scattered energy returning to the wellbore is meas-ured. Two detectors are used at different distances from the source so that a correlation for the effect of mud cake can be made. Gamma rays react with matter in these three ways: 1. Photoelectric absorption occurs for low energy gamma ray. The absorption depends on the atomic number of the nucleus and is the basis for the litholog (PE). 2. Compton scattering occurs over the entire energy spectrum and is the basis of the density log (FDC). The intensity at the diffused energy at the borehole wall is proportional to the bulk density.

WELL LOGGING AAPG ASUSC

LET’S LOGIN

After withdrawing the drilling ditch from the well, a sonde is lowered into the well to perform wireline logging (includes measurements of the formation’s electrical, nuclear, magnetic and acoustic properties). Open-hole logging refers to logging operations that are performed on a well before the wellbore has been cased and cemented. This is the most common type of logging method because the measurements are not obstructed and it's done during or after the well has been drilled. On the other hand, cased-hole logging involves retrieving logging measurements through the well casing, or the metal piping that is inserted into the well during completion operations. Cased-hole logging is performed more rarely but still provides valuable information about the well.

GAMMA-RAY LOG

NATURAL GAMMA-RAY LOGS Natural radiation is due to disintegration of nuclei in the subsurface. Potassium, Thorium and Uranium are the major decay series that contribute to natural radia-tion. Because K, Th and U tend to be concentrated in shales and are low or absent in clean sandstones and car-bonates, the gamma response is similar to the SP log. Open-hole and cased-hole gamma logs can also be cor-related and used to precisely locate pay zones for per-foration. Gamma-ray logs yield an approximate quantitative esti-mate of clay content or shaliness.

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ELECTRICAL LOGS Electrical sondes measure electrical properties in three different frequency ranges: 1. DC voltages that appear spontaneously in the wells (SP). 2. Strata and fluid resistivity (at low to medium frequencies 10 Hz to 20 kHz). 3. Dielectric constants (at high frequencies >10 MHz and up to 1 GHz). Dielectric logs give good results in low or variable salinity formation waters where resistivity methods have poor performance. Dielectric logs can be used where low gravity oils are not displaced. UHF logs have a very shallow investigation range; VHF logs investigate deeper.

RESISTIVITY LOGS Conventional resistivity logs were made by means of electrodes in contact with the formation through the drilling mud. There were several sondes capable of measuring to different distances (Short Normal, Long Normal and Lateral). Conventional logs gave good results in soft formations with fresh mud but the quality of the results declines in hard formations and carbonates. They have largely abandoned in favour of modern vertically or spherically focused logs and inductions tools. Guard logs or Laterlog produce results that are much less dependent on mud resistivity than conventional logs. These sondes have excellent vertical resolution to identify thin porous layers. The short guard or Laterlog 8 (LL8) is usually combined with the dual induction log. MICRORESISTIVITY LOGS Microresistivity logs are recorded on a small volume near a well filled with conductive mud. The aim is to determine the flushed zone resistivity ( Rₓₒ ) and the exact thickness of beds.

3. Electron-positron pairs are produced at relatively high energy. CALIPER LOG Caliper logs are required to assist in the quantitative interpretation of many other logs that are sensitive to borehole diameter and wall roughness (rugosity). Compensated logs such as density (FDC) and neutron (CNL) are corrected for these factors. The caliper shows where deviations occur from the nominal drill bit diameter. The deflections are towards smaller radius where mud cake has accumulated in porous formations and the oversize excursions where caving -washout- has taken place. Shales and coals are lithologies that tend to cave. The absence of mud cake adjacent to a porous bed may indicate a tight sand or possible overpressure.

DENSITY LOG If the grain density and the density of the mud filtrate are known, density logs give direct estimates of porosity (n). Mud filtrate has a density from 1.0 to 1.1 Mg/m³. n = (ρg - ρb )/(ρg - ρf) It is usual to calculate two porosities, one using a grain density of quartz (2.65 Mg/m³) and another using the density of calcite (2.71 Mg/m³). Dolomite has an even higher density (2.85 Mg/m³). Shale grain densities are in the range 2.4 to 2.6 Mg/m³. Assume the density log (FDC) indicates a bulk density of 2.2 Mg/m³ with a mud filtrate density of 1.1 Mg/m³, then porosity for sand and lime: n (sand) = (2.65 - 2.2) / (2.65 - 1.1) = 0.290 n (lime) = (2.71 - 2.2) / (2.71 - 1.1) = 0.317

The measuring device is mounted on a pad held against the well wall. The Microlog (ML), Microlaterlog (MLL), Proximity Log (PL) and Micro Spherically Focused Log (MSFL) are microresistivity sondes. The sondes are affected by mud thickness and the extend of the invaded zone. Microresistivity logs are not used for correlation but because they focus on very small volumes, they provide means for the very precise delineation of lithological boundaries. Microresistivity is used to estimate porosity assuming the flushed zone is saturated with mud filtrate.

INDUCTION LOGS

Induction sondes measure resistivity in wells drilled with non-conductive mud. They are focused to minimize the effect of the borehole and the invaded zone. Induction logs measure conductivity rather than resistivity. The DIL (Dual Induction Laterlog) system consists of a deep investigation induction sonde (ILd), a medium range induction sonde (ILm), a Laterlog 8 (LL8) and an SP electrode. The three focused resistivity readings can be used to accurately determine the true resistivity (Rt), even if the invaded zone is extensive. The formation resistivity is necessary to calculate porosity and fluid saturations using other logs.

SONIC LOGS Sonic logs (or acoustic) measure the porosity of the rock. Hence, they measure the travel time of an elastic wave through a formation (measured in ∆T- microseconds per meter). Intervals containing greater pore space will result in greater travel time and vice versa for non-porous sections. Must be used in combination with other logs, particularly gamma rays and resistivity, thereby allowing one to better understand the reservoir petrophysics.

AAPG ASUSC - LET’S LOG IN 3

Pₑ The litholog sonde records low energy gamma radiation arriving at the detector. The photoelectric absorption factor (Pₑ ) depends on the atomic number of the atoms in the formation and the PE log is sensitive to the composition of mineral phases. Because photoelectric effect is slightly affected by porosity and fluid content, the PE log is a direct indication of lithology.

MINERAL Pₑ

Quartz 1.81

Kaolinite 1.83

Montmorillonite 2.04

Dolomite 3.14

Illite 3.45

Halite 4.65

Anhydrite 5.05

Calcite 5.08

Chlorite 6.30

If the formation is gas-saturated (porosity calculated from density logs give anomalously high values since ρf for gas is 0.1 to 0.3 Mg/m³ and 1.0 to 1.1 was assumed.

NEUTRON LOGS Fast neutrons are emitted by a source in the sonde and travel through the formation where they are showed mainly by collision with hydrogen atoms. Slow neutrons are captured by atoms with the emission of gamma ray. Various logs detect: 1.Capture gamma ray 2. Slow (thermal) neutrons) 3. Partly slowed (epithermal) neutrons The Compensated Neutron Log (CNL) tool has two detector spacings and is sensitive to slow neutrons. The Dual Porosity CNL tool has two sets of detectors for both thermal and epithermal neutrons. CNL logs can be run in liquid-filled open-holes and cased-holes. There are several single-detector, pad-type neutron tools that use epithermal detectors. These include the Sidewall Neutron Porosity (SNP).

Well logging vs. Formation evaluation vs. Petrophysics

Well logging or ‘borehole logging’ is making a detailed record (a well log) of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface (geological logs) or on physical measurements made by instruments lowered into the hole (geophysical logs) Formation evaluation is to determine the ability of a borehole to produce petroleum. Essentially, it is the process of "recognizing commercial wells. Petrophysics is the study of physical and chemical rock properties and their interactions with fluids. A major application of petrophysics is in studying reservoirs for the hydrocarbon industry.

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